Plasma polymerization, sometimes designated "plasma enhanced chemical vapor deposition," or "PECVD," has been a known technique to form films on various substrates. For example, mixtures of silane with or without oxygen, nitrous oxide or ammonia have been plasma polymerized to form silicon oxide films. Sacher et al., U.S. Pat. No. 4,557,946, issued Dec. 10, 1985 describes use of plasma polymerized coatings from organosilicon compounds to form a moisture barrier on the substrate by heating the substrate and controlling the plasma power level. Wertheimer et al., U.S. Pat. No. 4,599,678, issued Jul. 8, 1986, discloses use of an organosilicon in a glow discharge to coat thin film capacitors when these substrates are heated to a temperature in excess of 50.degree. C.
In general, the films formed from organo-silicons have typically been formed at a relatively low deposition rate (as compared with, for example, magnetron sputtering), have tended to be soft, and often have been hazy. The requirement that the substrate be heated, as in Sacher et al. and Wertheimer et al., is also disadvantageous for some substrates.
A further problem with use of organosilicon compounds in plasma enhanced deposition has been the variation in polymerization conditions and lack of control during the deposition. The traditional method used to control plasma processes has been the use of power, pressure and flow to monitor and attempt to control the process. However, these three variables represent inputs and do not accordingly control the thin films being produced. As a consequence, the scale-up of such a process is extremely complex.
In early microelectronic PECVD reactors, the plasma was generated between two parallel, circular electrodes. The wafers were loaded onto the lower, electrically grounded, electrode. The upper electrode was connected to a rf generator through an impedance matching network. The reactants were fed in from a gas ring, entered the plasma region (i.e., the region between the electrodes) at its outer edge, and flowed radially in toward a pumping port at the center of the electrode. These reactors have commonly been known as "radial flow" reactors.
In "inverse" radial flow reactors, the gas inlet has been at the center of the lower electrode, with the gas flow directed radially outward. A magnetic drive assembly permitted rotation of the lower electrode, thus randomizing the substrate position and optimizing deposition uniformity.
In hot-wall, batch PECVD systems, the deposition chamber consisted of a quartz tube placed within a resistively heated furnace. Vertically oriented graphite slabs carried the wafers in slots. Every other slab was connected to the same rf power terminal and a glow discharge was generated between adjacent electrodes. The reactants were directed along the axis of the chamber tube and between the electrodes.
More recently, PECVD has been employed to coat large substrates, e.g., plastic containers and long rolls of flexible films for food packaging applications. In the process described by Ser. No. 07/426,514, filed Oct. 24, 1989, of common assignment herewith, plasma polymerization is used to deposit silicon oxide based thin films from volatile organosilicon compounds. This method of depositing an adherent, hard silicon oxide based film comprises providing a gas stream with several components, establishing a glow discharge plasma derived from the gas stream, or one of its components, in a previously evacuated chamber with a substrate removably positioned in the plasma, and controllably flowing the gas stream into the plasma to deposit a silicon oxide onto the substrate when positioned in the plasma. The gas stream components include a volatilized organosilicon compound, oxygen, and an inert gas such as helium or argon.
The gas stream is controllably flowed into the plasma by volatilizing the organosilicon exterior to the chamber and admixing metered amounts of organosilicon with oxygen and the inert gas. Controlling the flowing gas stream into the plasma preferably includes adjusting the amount of organosilicon entering the chamber during the deposition. Control can be achieved with use of the flow vaporizer described by U.S. Pat. No. 4,847,469, issued Jul. 11, 1989, inventors Hofmann et al.
Films with reduced permeability to vapors such as water, oxygen, and carbon dioxide are useful for a variety of applications, one of which is to package foods. Such films are typically composites of materials. For example, one layer is often a flexible polymer, such as a polyethylene or polypropylene, while another layer is coated on or coextruded with the one layer and serves as a barrier layer. Barrier layers can generally be viewed as substantially organic based or substantially inorganic based.
Plasma assisted or enhanced processes, in addition to coating applications such as above described, include plasma etching or cleaning where substrate surfaces are modified. Plasma etching, for example, is used in manufacturing integrated electronic circuits.
A variety of equipment for plasma treatments are known. U.S. Pat. No. 4,863,756, issued Sep. 5, 1989, inventors Hartig et al., describes plasma coating equipment that includes magnets disposed to one side of an electrode while the other electrode side holds the substrate being coated, which faces a plasma.
U.S. Pat. No. 4,968,918, issued Nov. 6, 1990, inventors Kondo et al., discloses a plasma treating apparatus having a plurality of powered electrodes. The substrate being plasma treated is passed proximate to the powered electrodes.
U.S. Pat. No. 5,009,738, issued Apr. 23, 1991, inventor Gruenwald, discloses a plasma etching apparatus in which the substrate being treated is pressed against a cathode and the apparatus is said to achieve an improved heat elimination for the substrate during the etching process.
U.S. Pat. No. 5,013,416, issued May 7, 1991, inventor Murayama, discloses an apparatus for manufacturing transparent, conductive film by using an ion-plating method with a pressure-gradient plasma gun and intermediate electrodes for plasma control. The apparatus is said to permit manufacture of conductive films at high substrate velocity.